The present invention relates to a hydraulic vehicle brake system with anti-locking, wherein a braking pressure generator comprising a hydraulic power booster and a master cylinder connected downstream thereof is employed for the pressure supply of the brake circuits. Wheel brakes are connected to the pressurizable working chambers of the braking pressure generator by way of first valves. The working chambers are connectible with an auxiliary pressure source by way of second valves. A third valve is interposed between the auxiliary pressure source and the second valves which can be switched to assume a closed position in response to a pedal contact and the pressure of the auxiliary pressure source.
In German patent application No. P 34 27 070.1, a brake system of the type referred to is described, wherein a braking pressure generator is used which is substantially composed of a hydraulic power booster and a tandem master cylinder connected downstream thereof. Basically, the auxiliary pressure source consists of an electromotively driven pressure fluid pump which serves to charge a pressure fluid accumulator having a limited storage volume to a predeterminable pressure level during normal driving operation (i.e., without braking). At the outlet of the auxiliary energy source, a valve is connected which establishes a hydraulic connection between the auxiliary energy source and the working chambers of the tandem master cylinder when a predetermined pressure prevails at the outlet of the auxiliary energy source. This connection can be established by virtue of sealing sleeves which are arranged between end faces of the master cylinder pistons close to the pedal and the working chambers of the tandem master cylinder and which are caused to assume their opened position in the presence of a corresponding pressure gradient.
The brake system described is more specifically designed for a dual-circuit brake system, wherein to each working chamber of the tandem master cylinder one brake circuit is connected. A component of the valve assembly is a piston which sets the valve mechanism into operation and which can be acted upon by the pressure built up in the pressure chamber of the hydraulic power booster. Inserted between the outlet of the auxiliary energy source and the valve connected upstream of the master cylinder is a so-called safety valve which normally assumes its closed position and which is switched to its opened position on brake application.
The hydraulic power booster of the known brake system includes a brake valve which, when the brake pedal is applied by force, permits development of a pressure in the pressure chamber of the hydraulic power booster which is proportional to the force exerted on the brake pedal. Furthermore, when the brake pedal is depressed, a brake pedal contact will be made, as a result whereof the pressure fluid pump is put into operation by way of an electric motor. In the initial phase of brake actuation, the pressure fluid volume necessary for the hydraulic power boosting is taken from a pressure fluid accumulator with limited storage volume, whereby the start-up period or, respectively, the rate of delivery of the pressure fluid pump which is reduced in the start-up period will be compensated.
On depression of the brake pedal, a safety valve also will assume its opened position, the safety valve being arranged between the outlet of the pressure fluid pump an the valve. Simultaneously, the pressure which developed in the pressure chamber of the hydraulic power booster pressurized a piston of the valve which, due to this pressurization, displaces such that the end surfaces of the valve close to the pedal are isolated from the unpressurized supply reservoir. In the event of a pressure increase at the outlet of the auxiliary energy source, valve passages within the valve will adopt their opened position, so that the end surface of the master cylinder pistons close to the pedal are connected to the outlet of the auxiliary energy source.
It is a disadvantage in the brake system described that the system is designed for a dual-circuit brake system, each individual brake circuit being connected to one working chamber of the tandem master cylinder. Further, upon the occurrence of leakage in one circuit, it will not be possible without further efforts to perform a brake slip control action in the other brake circuit.
It is therefore an object of the present invention to improve upon a hydraulic vehicle brake system with anti-locking of the type referred to in such a manner that the brake system can be designed to comprise three circuits, on the one hand, and that the anti-locking function is preserved, on the other hand, if any one of the brake circuits connected to the braking pressure generator fails.